1. Field of the Invention
The present invention relates generally to glass substrates, and particularly to a glass substrate product for use in passive or active display manufacturing processes.
2. Technical Background
Liquid crystal displays (LCDs) are non-emissive displays that use external light sources. An LCD is a device that may be configured to modulate an incident polarized light beam emitted from the external source. LC material within the LCD modulates light by optically rotating the incident polarized light. The degree of rotation corresponds to the mechanical orientation of individual LC molecules within the LC material. The mechanical orientation of the LC material is readily controlled by the application of an external electric field. This phenomenon is readily understood by considering a typical twisted nematic (TN) liquid crystal cell.
A typical TN liquid crystal cell includes two substrates and a layer of liquid crystal material disposed there between. Polarization films, oriented 90° one to the other, are disposed on the outer surfaces of the substrates. When the incident polarized light passes through the polarization film, it becomes linearly polarized in a first direction (e.g., horizontal, or vertical). With no electric field applied, the LC molecules form a 90° spiral. When incident linearly polarized light traverses the liquid crystal cell it is rotated 90° by the liquid crystal material and is polarized in a second direction (e.g., vertical, or horizontal). Because the polarization of the light was rotated by the spiral to match the polarization of the second film, the second polarization film allows the light to pass through. When an electric field is applied across the liquid crystal layer, the alignment of the LC molecules is disrupted and incident polarized light is not rotated. Accordingly, the light is blocked by the second polarization film. The above described liquid crystal cell functions as a light valve. The valve is controlled by the application of an electric field. Those of ordinary skill in the art will also understand that, depending on the nature of the applied electric field, the LC cell may also be operated as a variable light attenuator.
An Active Matrix LCD (AMLCD) typically includes several million of the aforementioned LC cells in a matrix. Referring back to the construction of an AMLCD, one of the substrates includes a color filter plate and the opposing substrate is known as the active plate. The active plate includes the active thin film transistors (TFTs) that are used to control the application of the electric field for each cell or subpixel. The thin-film transistors are manufactured using typical semiconductor type processes such as sputtering, CVD, photolithography, and etching. The color filter plate includes a series of red, blue, and green organic dyes disposed thereon which ideally corresponds precisely with the subpixel electrode area of the opposing active plate. Thus, each sub-pixel on the color plate should be aligned with a transistor controlled electrode disposed on the active plate, since each sub-pixel must be individually controllable. One way of addressing and controlling each sub pixel is by disposing a thin film transistor at each sub pixel.
The properties of the aforementioned substrate glass are extremely important. The physical dimensions of the glass substrates used in the production of AMLCD devices must be tightly controlled. The fusion process, described in U.S. Pat. No. 3,338,696 (Dockerty) and U.S. Pat. No. 3,682,609 (Dockerty), is one of the few processes capable of delivering substrate glass without requiring costly post-substrate forming finishing operations, such as lapping, grinding, and polishing. Further, because the active plate is manufactured using the aforementioned semiconductor type processes, the substrate must be both thermally and chemically stable. Thermal stability, also known as thermal compaction or shrinkage, is dependent upon both the inherent viscous nature of a particular glass composition (as indicated by its strain point) and the thermal history of the glass sheet, which is a function of the manufacturing process. Chemical stability implies a resistance to the various etchant solutions used in the TFT manufacturing process.
There is a demand for ever larger display sizes. This demand, and the benefits derived from economies of scale, are driving AMLCD manufacturers to process larger sized substrates. When assembled, components on each side of the two substrates, or sub-sheets, used to form the display must align precisely during assembly. Pixel misalignment by as little as 2% is visually detectable, and therefore unacceptable.
Unfortunately, stresses which may be frozen into the glass sheets during manufacture of the parent sheet may result in distortion of the sub-sheets after the parent glass sheet is cut. This distortion is exacerbated as the size of the sheet is increased. However, such future distortion is not easily discerned in the parent glass as manufactured by the glass manufacturer.
What is needed is a method of equating stresses within the parent sheet of glass to distortion which may be exhibited by an individual sub-sheet sheet when the parent sheet is cut.